1. Field of the Invention
This invention relates generally to bleeding air from a fan or compressor of gas turbine engines and, more particularly, to bleeding a wide range of variable fraction of air from a fan or compressor of a gas turbine engine.
2. Discussion of the Background Art
Bypass gas turbine engines typically include an axial flow fan and a high pressure compressor which supplies high pressure air to a combustor. The fan and compressor typically includes multiple stages. Each stage is composed of a stationary component referred to as a stator and a rotational component, which adds work to the system, referred to as a rotor. A portion of compressed interstage air may be extracted from either component for turbine section cooling, airframe pressurization, anti-icing, and other uses.
Conventional bleed systems extract a relatively small and relatively fixed fraction of fan or compressor air through bleed openings between stator vanes. The fan and/or compressor stages downstream of the bleed openings are accordingly sized and designed to minimize performance penalty to an engine cycle. There is a need to provide a bleed system for efficiently supplying a large variable amount of bleed air at various flight conditions while minimizing performance penalty to an engine cycle. In a non complementary manner, the bleed system has to be sized and designed for a maximum bleed air flow requirement and the fan and/or compressor sections downstream of the bleed openings have to be sized to accept 100 percent fan flow with little or no bleed flow being extracted.
Therefore, it is desirable to provide for bleeding or extraction of a variable percentage of interstage fan air at various flight conditions, with little or no adverse performance effect or performance penalty to an engine cycle.
An air bleed assembly for extracting air from a flowpath in a gas turbine engine includes a casing for surrounding a row of circumferentially spaced apart rotor blades and defining the flowpath for receiving air compressed by the rotor blades. The casing includes a bleed port disposed downstream of a row of the blades for receiving a portion of the compressed air as bleed airflow and a bleed duct extending away from the bleed port. The bleed duct has an annular slot in the casing. The annular slot has annular slot leading and trailing edges and an annular bifurcated splitter disposed along at least a portion of the annular slot trailing edge. The bifurcated splitter has an annular leading edge forebody located upstream of and separated from an annular splitter wall by an annular return channel.
In the exemplary embodiment of the invention, the leading edge forebody has an airfoil shaped cross-section with a radially outwardly facing suction side and a radially inwardly facing pressure side. The leading edge forebody is an annular ring and the splitter wall has a blunted leading edge annularly bounding return channel. The leading edge forebody is supported by support vanes extending through at least a portion of the bleed duct. Channel vanes extend through the bleed duct aft of the support vanes. At least one bleed plenum is in fluid communication with the bleed duct and first and second bleed air circuits are in downstream fluid communication with the bleed plenum. First and second control valves are disposed in the bleed circuits downstream of the plenum. An annular bleed space in the flowpath is located between a vane, that may be variable, disposed across the flowpath and one of the rotor blades located downstream of the variable vane. The annular slot and the annular bifurcated splitter are located along the annular bleed space.
In a more particular embodiment of the invention, the rotor blades are fan blades and the annular bleed space is in the flowpath in a fan section of the engine. The annular bleed space is located between a fan variable vane disposed across the flowpath and one of the fan blades located downstream of the fan variable vane.